Regarding a construction machine, there is known a technique regenerating the return hydraulic fluid from the hydraulic actuator via a control valve in order to improve the fuel efficiency of the engine and to attain energy saving. Examples of the technique are disclosed in Patent Document 1 and Patent Document 2.
Patent Document 1 discloses a hydraulic control system in which a power discharged from a bottom side hydraulic fluid chamber when a boom cylinder for driving a work device of a construction machine falls due to its own weight is regenerated for the driving of another hydraulic actuator via a control valve.
Patent Document 2 discloses a hydraulic drive system in which a high pressure hydraulic fluid in a rod side hydraulic fluid chamber of a boom cylinder is regenerated in a bottom side hydraulic fluid chamber of an arm cylinder at the time of excavating when the hydraulic excavator performs a combined operation of boom raising and arm crowding in order to efficiently utilize the return hydraulic fluid, which is discharged to a tank in the prior art.